The term "transparent soap" as used herein encompasses soaps having a wide degree of color and gloss but which are sufficiently transluscent so that one can effectively see through a toilet sized bar. For example, if 14 point type can be read through a 1/4" bar of soap, the soap can be regarded as transparent.
A variety of transparent soaps have been formulated. A common technique has been based upon the addition of a polyhydric alcohol such as glycerol, glycol, sugar or the like to a "neat soap" or semi-boiled soap, or to soap prepared by the cold process technique. Another method consists of dissolving soap in alcohol to remove saline impurities and then distilling off most of the alcohol. U.S. Pat. No. 3,562,167 describes a transparent soap formed from a combination of soap, polyhydric alcohol and, as a surface active agent, a polyalkoxy ether of an alkylphenol. U.S. Pat. No. 3,903,008 describes the formulation of a transparent soap by the combination of soap, polyhydric alcohol and an amphoteric imidazolene detergent. U.S. Pat. Nos. 3,793,214 and 3,926,808 describe transparent soaps produced using branched chain fatty acids. U.S. Pat. No. 3,864,272 describes the use of rather complicated, elaborate mechanical methods of working the soap.
A method of formulating a transparent soap is disclosed in U.S. Pat. No. 2,820,768 where a sodium soap made from tallow, coconut oil and castor is mixed with a triethanolamine soap of stearic acid and oleic acid and an excess of the amine. Small changes in the amount of amine component or of the relative proportions of certain ingredients leads to loss of transparency.
Low alkalinity is a desirable feature of toilet soaps and some current transparent soaps possess this feature. When the soap is an alkaline metal salt of a long chain high molecular weight carboxylic acids, it will have a pH of about 10 even if there is no free titratable alkali present in the solid soap. It is known that the addition of an alcohol amine, such as triethanolamine, to a soap results in a substantially non-alkaline soap; a soap having a pH of 7.5 to 9.0 in 10 weight percent aqueous solution can be considered to be substantially non-alkaline.
To be commercially acceptable, a transparent soap must have good bar soap characteristics, such as lathering, firmness, hardness, mildness, minimum slushing, low background odor, and safety in use. The soap must keep its transparency under all types of aging conditions.
The present invention resides in the discovery that another class of amine can be used in formulating a substantially non-alkaline, solid transparent soap. Certain preferred ratios of components are critical, albeit different, as with formulations based on the use of triethanolamine.
Specifically, the particular amine class that is useful herein is a tetrakis (hydroxyalkyl) ethylene diamine having the formula: ##STR2## wherein R is hydrogen or an alkyl group having one to four carbon atoms and n is one to four. The foregoing diamine has a molecular weight of under 1700. It is combined with a transparent sodium soap prepared by saponification of fatty oil and a polyhydric alcohol as solvent.
More particularly, one mixes together caustic soda (i.e., sodium hydroxide), saponifiable fatty oil to react with the caustic soda to form a soap, water and a polyhydric alcohol. The diamine can be added before or after saponification. After saponification, one must add a superfatting agent, preferably one or more fatty acids of C.sub.12 -C.sub.18, both fully saturated and unsaturated, straight or branched. Examples include stearic acid, oleic acid, isostearic acid, fatty acids derived from tallow oil or coconut oil, i.e. tallow fatty acid, hydrogenated tallow fatty acid, coconut fatty acid, and the like. Particularly preferred is stearic acid optionally with oleic acid. Other components are those adjuvants that are known to the art including: a humectant such as glycerine, foam boosters and stabilizers, surfactants, chelating compounds, and perfume. The saponifiable fatty oil is preferably a mixture of tallow, coconut oil and caster oil in certain defined ratios, as will be hereinafter described. The mixture is agitated and heated until it is well mixed.